Frequency modulation phonograph system



Dec. 6, 1949 M. G. NICHOLSON, JR 2,490,277

FREQUENCY MODULATION PHONOGRAPH SYSTEM Filed Dec. 22, 1945 2 Sheets-Sheet 1 (Iffornqr) 1949 v M. s. NICHOLSON, JR 2,490,277

FREQUENCY MODULATION.PHONOGRAPH SYS'f'EH Filed D60. 22, 1945 2 Sheets-Sheet 2 Fe; W

. mmvzm Patented Dec. 6, 1949 2,490,277 FREQUENCY moiggi a'rlon PHONOGRAPH T Madison G. Nicholson, J r., Snyder, N. Y., assignor to Colonial Radio Corporation, Buflalo, N. Y.

Application December 22, 1945, Serial No. 636,715

Claims.

This invention relates to electrical phonographs, and more especially to electrical phonographs employing a capacity pickup, and utilizing the 'output of the pickup to provide a frequency modulated radio frequency oscillation.

Systems of this type as heretofore proposed and used suffer from a number of defects. Among these may be mentioned the inability to obtain a high percentage of modulation, with resultant inefficiency and undesirably high ratios of noise to signal.

In attempting to overcome these defects, the circuits employed have been unnecessarily complicated and expensive, with the result that, to obtain comparable performance from an F. M. phonograph system to that of an A. M. phonograph system, the cost of the F. M. system is much greater than that of the A. M. system. If comparisons are made on apparatus of the same cost, the performance and efficiency of the F. M.

system is much less than that of the A. M.

system.

It is an object of this invention to improve the efliciency and performance of F. M. phonograph systems, while at-the same time maintaining the apparatus simple and its cost comparable with that of an A. M. system capable of the same performance.

It is a further object of this invention to provide an F. M. phonograph system in which a relatively high percentage modulation can be obtained with simple and inexpensive apparatus.

It is still a further object of this invention to provide an F. M. phonograph system capable of high percentage modulation and providing a high ratio of signal to noise.

It is a further object of this invention to provide an F. M. phonograph system in which the expensive filters heretofore employed for reduction of noise are eliminated.

It is a further object of this invention to provide a generally improved, more efilcient, and simple F. M. phonograph system.

Still other objects and advantages of my invention will be apparent from the specification.

The features of novelty which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its fundamental principles and as to its particular embodiments will best be understood by reference to the specification and accompanying drawing, in which Fig. l is a circuit diagram of a simple form of oscillator employed in accordance with my invention for producing frequency modulated oscillations, and

Figs. 2, 3, and 4 are variant forms of such an oscillator employing a capacity phonograph pickup and certain other features in accordance with my invention.

Referring now more particularly to Fig. l, I prefer to employ as an oscillator thermionic vacuum tube l0, preferably a pentode, having relatively high mutual conductance. This tube in the usual form may comprise cathode I00, heater "in, control electrode Hlg, screen electrode Illsc, suppressor electrode lllsr, and anode Illa.

The cathode may be directly grounded and control electrode Illg connected to cathode through resistance R5. The screen electrode may be by-passed to cathode I00 by means of condenser C8 and may be connected to +3 through resistor R6. Suppressor lllsr may be connected directly to cathode Illc. The plate circuit may include a tuning circuit, the frequency of which is determined by capacity Cl, inductance LI, and capacity C2.

In the arrangement shown, it is preferred to ground one side of condenser Cl and to connect inductance L! to condenser C2 through condenser C6, the latter being merely a blocking condenser which plays no part in the frequency determination of the oscillation. Plate voltage may be applied through resistor R! and inductance LI to anode Illa, and inductance Ll may be provided with a variably mounted permeability core for permeability tuning. The source of plate voltage thus may be a battery or the like, the negative side of which may be grounded as usual.

The grid circuit may be tuned by the provision of inductance L4, like Ll, permeability tuned, shunted by condenser C4, both the latter bein connected in series with condenser C5 between control electrode lllg and ground. Resistance R5 is preferably made sufiiciently small to limit the selectivity of the grid circuit.

It may be noted that in this system it is undesirable to have the tuned circuits too selective, because this tends to limit the degree of modulation obtainable, and in this respect better results are obtained with circuits which are not too sharply tuned.

A feedback path may be provided by connecting suitable resistance R3 between points Fp in the output circuit and Fg in the input circuit. This gives the optimum phase feedback for oscillation when the plate and grid circuits are both tuned to the desired frequency of oscillation, because the plate circuit from anode lfla to point Fp introduces a phase shift of 180 and tube It introduces a phase shift of 180 between the grid cathode and the anode-cathode points, this making a net phase shift in the complete circuit of 350 or If R3 is shunted by a small value of capacity, the frequency of oscillation will change. This is because the introduction of the additional capacity introduces an additional phase displacement, and the frequency of oscillation changes until at the new frequency the net phase relation is 360 01' 0.

From this it follows that the connection of a variable capacity in shunt with R3 will vary the generated frequency as the capacity varies and, as already pointed out, the lower the selectivity of the grid and plate tuned circuits, the greaterwill be the frequency shift for a given value or change in value of a small value of capacity across R3.

By using a high mutual conductance tube in the circuit, the tuned circuits may be damped with low values of resistance such as RI and R5 to reduce the selectivity and still maintain oscillation. Thus, the higher the mutual conductance of the tube, the greater is the frequency deviation or modulation obtainable with this systern.

Referring now more particularly to Fig. 2, I have shown the application to the oscillator of Fig. 1 of a three-terminal capacity pickup network described and claimed in my co-pendinz application entitled Capacity microphone, pickup, etc., operating circuits," Serial Number 635,- 275, filed December 15, 1945, in which such a pickup network is employed for feeding the input of an audio frequency amplifier and also for producing amplitude modulation of radio frequency oscillations. In this application such a network is applied to the oscillator shown in Fig. 1 hereof for the production of frequency modulated oscillations.

In Fig. 2, wherein like reference numerals designate like parts, as in Fig. 1, C3 designates a phonograph capacity pickup or a condenser microphone. This differs from conventional practice in that neither electrode of the capacity element is grounded, one being connected to the common point of R3 and capacitance C5, and the other electrode being connected to the common point of R3 and capacitance C6 so that capacity C3 is in shunt with resistance R3.

The conductors which lead from the respective electrodes of capacity C3 to the common points above identified are preferably enclosed within a shielded and grounded cable, and the capacity of the left-hand cable seen in Fig. 2 replaces capacity C4 of Fig. 1, while that of the righthand cable replaces capacity C! of Fig. 1.

The only difference between Fig. 1 and Fig. 2 is the addition of capacity Cl, connected between the common point of resistance R3 and condenser C5, and anode "a, which may be provided in case the residual capacity of pickup C3 is too high and the frequency deviation is thereby reduced. The provision of capacity C1 increases the frequency deviation obtainable.

Condenser C! may have a value approximately equal to the residual capacity of pickup C3 multipiied by ratio Cl/CI, where Cl is the capacity of condenser Cl and C2 is the effective capacity of shielding cable C2 to the respective pickup connection. In this circuit the capacitance of the two shielded leads to the pickup elements are fractions of, or the total of, capacity C2 and Cl respectively.

A variation of the above described circuit is shown in Fig. 3, wherein like reference numerals indicate like parts, as in Fig. 2 and Fig. 1. In this instance inductance L3 replaces resistor R1. Inductance L3 may be associated with a variably mounted permeability tuning core, and the parallel resonant resistance of inductance L3 is equivalent to R3.

The action of the inductance is to neutralize the eifect of the residual capacity of pickup Cl and, therefore, condenser I, provided in Fig. 2 to neutralize this residual capacity, may be omitted. This circuit is not as effective as the circuit of Fig. 2, bcause inductance L3 does have an appreciable distributed capacity which is efiectively in parallel with pickup C3.

Referring now to Fig. 4, this differs from the circuit of Fig. 2 in that the phase shift network has been moved to the grid circuit from the plate circuit, thereby eliminating the need for coupling capacitor C5. It will be noted that condenser CI of Fig. 2 is now connected from screen grid to ground in Fig. 4, and renumbered in the new position as C9, and that inductance L4, instead of being connected to ground at one end, now connects directly to control electrode Ila on the other end, and to the common point of the pickup electrode and resistance R3.

Also, it will be observed that resistance RI has been omitted in Fig. 4, capacity C6 is now connected between the common point of resistance R3 and the pickup electrode on the one hand and to anode Illa on the other, and capacity C! is now connected from control electrode lily to the common point of resistance R3 and capacity C6. Otherwise the operation of the circuit of Fig. 4 is the same as that described for Fig. 2.

Although for simplicity of description I have shown and described the use of shielded conductors connected to both electrodes of the capacity pickup or microphone, it will be understood that satisfactory operation may be obtained with either lead shielded, and the other unshielded. Also, effective shielding may be accomplished without the use of ordinary shielded cable, by placing the leads close to grounded conductors, metal parts, or the like, and by keeping the leads apart, especially where they are not placed close to metallic parts. As an example of this, the leads to C3 may be shielded by the metallic tone arm of the phonograph, even though this is not completely closed (such tone arms are usually channel shaped in cross-section, and open on the bottom), and where they leave the tone arm, they should run oil in opposite directions, as nearly as possible.

In the specification I have explained the principles of my invention and the best mode in which I have contemplated applying those principles, so as to distinguish my invention from other inventions; and I have particularly pointed out and distinctly claimed the part, improvement, or combination which I claim as my invention or discovery.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from the spirit and scope thereof, as will be clear to those skilled in the art.

I claim:

1. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode containing an impedance connected in a circuit external to said tube, an electro-mechanical translating device having a pair of capacity electrodes connected in shunt with said impedance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by said electromechanical translating device, said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereof.

2: A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode containing an impedance connected in a circuit external to said tube, an electro-mechanical translating device having a pair of capacity electrodes connected in shunt with said impedance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by said electromechanical translating device, and said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereto.

3. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode consisting of a resistance connected in a circuit external to said tube, an electro-mechanical translating device having a pair of capacity electrodes connected in shunt with said resistance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by said electromechanical translating device, and said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other comiection thereto.

4. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode consisting of a resistance connected in a circuit external to said tube, an electro-mechanical translating device having a pair of capacity electrodes, both maintained at a potential, connected in shunt with said resistance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by the said electro-mechanical translating device, and said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereto.

5. A frequency modulation system comprising,

in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at

least one tuned circuit connected between the phase of the voltage delivered by said electrotil mechanical translating device, and said latter two voltages producing sustained oscillation and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereto.

6. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode containing an inductance connected in a circuit external to said tube, an electro-mechanical translating device having a pair of capacity electrodes connected in shunt with said inductance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by said electromechanical translating device, and said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereto.

7. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode containing an impedance connected in a circuit external to said tube, and

a capacity microphone having a pair of capacity electrodes connected in shunt with said impedance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by said capacity microphone, and said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereto.

8. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode containing an impedance connected in a circuit external to said tube, a condenser microphone having a pair of capacity electrodes maintained at a potential connected in shunt with said impedance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by said condenser microphone, and said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereto.

9. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the 7 cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode containing an impedance connected in a circuit external to said tube, a phonograph pickup'of the capacity type having a pair of capacity electrodes connected in shunt with said impedance, said feedback path delivering a voltage having a phase different from the phase of the voltage delivered by said phonograph pickup, and said latter two voltages producing sustained oscillation, and a metallic conductor shielding one connection to said capacity electrodes from the other connection thereto.

10. A frequency modulation system comprising, in combination, a thermionic vacuum tube having a cathode, a control electrode, and an anode, at least one tuned circuit connected between the cathode and one of the other electrodes of said tube, a feedback path between said anode and said control electrode containing an impedance connected in a circuit external to said tube, a phonograph pickup of the capacity type having a pair of capacity electrodes maintained REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,219,030 Curtis Oct. 22, 1940 2,288,575 Stablein June 30, 1942 2,319,622 Miessner May 18, 1943 2,361,634 Koch Oct. 13, 1944 2,371,373 Badmaieif Mar. 13, 1945 

